The presence of large deposits of oil shale in the semi-arid, high plateau region of the western United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising a marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products, including hydrocarbon products. It is the formation containing kerogen that is called "oil shale" herein and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either mining the kerogen-bearing shale and processing the shale on the surface or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes. According to both of these approaches, oil shale is retorted by heating the oil shale to a sufficient temperature to decompose kerogen and produce shale oil which drains from the rock. The retorted shale, after kerogen decomposition, contains substantial amounts of residual carbonaceous material which can be burned to supply heat for retorting.
One technique for recovering shale oil includes forming an in situ oil shale retort in a subterranean formation containing oil shale. At least a portion of the formation within the boundaries of the in situ oil shale retort is explosively expanded to form a fragmented permeable mass of particles containing oil shale. The fragmented mass is ignited near the top of the retort to establish a combustion zone. An oxygen-supplying gas is introduced into the top of the retort to sustain the combustion zone and cause it to move downwardly through the fragmented permeable mass of particles in the retort. As burning proceeds, the heat of combustion is transferred to the fragmented mass of particles below the combustion zone to release shale oil and gaseous products therefrom in a retorting zone. The retorting zone moves from the top to the bottom of the retort ahead of the combustion zone and the resulting shale oil and gaseous products pass to the bottom of the retort for collection and removal. Recovery of liquid and gaseous products from oil shale deposits is described in greater detail in U.S Pat. No. 3,661,423 to Donald E. Garrett.
As used herein, the term "retorting zone" refers to that portion of the retort where kerogen in oil shale is being decomposed to liquid and gaseous products, leaving residual carbonaceous material in the retorted oil shale. The term "combustion zone" refers to a portion of the retort where the greater part of the oxygen in the retort inlet mixture that reacts with the residual carbonaceous material in the retorted oil shale is consumed.
It has been found desirable in some embodiments to have an intact subterranean base of operation above the fragmented permeable mass of formation particles in an in situ oil shale retort. Such a base of operation facilitates the drilling of blastholes into underlying formation for forming the fragmented mass in the retort and facilitates ignition over the entire top portion of the fragmented mass. Additionally, having a base of operation above the fragmented mass permits control of introduction of oxygen-supplying gas into the retort, provides a location for testing properties of the fragmented mass, such as distribution of void fraction, and provides a location for evaluation and controlling performance of the retort during operation.
The base of operation is separated from the retort by a layer of unfragmented formation extending between the top boundary of the retort and the floor of such a base of operation. The layer of unfragmented formation is termed a "sill pillar" which acts as a barrier between the in situ oil shale retort and the base of operation during retorting operations. It is, therefore, important that the sill pillar remain structurally sound, both for supporting the base of operation and for preventing entry of heat and gases into the base of operation during the retorting process.
Techniques for forming an in situ oil shale retort containing a fragmented permeable mass of formation particles and having a sill pillar of unfragmented formation between the top of the fragmented mass and an overlying base of operation are described in U.S. Pat. No. 4,118,071 by Ned M. Hutchins and in U.S. Pat. No. 4,192,554 by Thomas E. Ricketts. U.S. Pat. Nos. 4,118,071 and 4,192,554 are incorporated herein by this reference. The in situ oil shale retort formed by the method disclosed in U.S. Pat. No. 4,192,554 may not be completely full of oil shale particles, i.e., there can be a void space between the upper surface of the fragmented mass of oil shale particles and the top boundary of the retort.
In other retorts where no open base of operation is provided, the formation overlying the fragmented permeable mass of formation particles extends all the way to the ground surface. In such an embodiment, blastholes are drilled through the overlying formation and ignition of the fragmented mass of particles is accomplished from the ground surface.
Examples of other techniques used for forming in situ oil shale retorts are described in U.S. Pat. No. 4,043,595 by French; U.S. Pat. No. 4,043,596 by Ridley; U.S. Pat. No. 4,043,597 by French; and U.S. Pat. No. 4,043,598 by French et al, each of which is incorporated herein by this reference.
In the past, a variety of techniques have been developed for igniting oil shale particles in an in situ oil shale retort in order to establish a combustion zone. Such techniques are disclosed in U.S. Pat. No. 3,952,801 and U.S. Pat. No. 3,990,835, both by Robert S. Burton, III. According to the techniques disclosed in these patents, a hole is bored to the top of the fragmented permeable mass of oil shale particles and a burner is lowered through the borehole to the oil shale to be ignited. A mixture of combustible fuel, such as LPG (liquefied petroleum gas), diesel oil, or shale oil, and oxygen-containing gas, such as air, is burned in the burner to provide a hot ignition gas which is introduced into the fragmented mass of oil shale particles. The burning is continued until a substantial portion of the oil shale has been heated above its self-ignition temperature so that combustion of the oil shale in the fragmented mass is self-sustaining after ignition. Thereafter, the burner is extinguished and an oxygen-supplying gas is introduced into the retort to advance the combustion zone through the fragmented mass.
Alternatively, if desired, a lateral drift which communicates with the top region or surface of the fragmented mass can be formed through a side boundary of the retort and hot ignition gases can be introduced through the drift.
It can be important for several reasons to minimize the amount of time it takes to complete ignition of the fragmented mass in the retort.
For example, until ignition is completed, effective retorting does not take place and, thus, no products are recovered. In addition, when a void is between the overlying unfragmented formation and the fragmented mass, heating of the overlying formation during ignition can result in spalling or sloughing of the formation into the retort. This can prolong the ignition process and, in some instances, can make ignition impossible. Additionally, the sloughed formation can be heated sufficiently to consume at least a portion of the oxygen being supplied to the retort during retorting operations. This can upset the desired material balance in the retort and deleteriously affect the amount of products recovered.
If ignition time were minimized in the first instance, heating of overlying formation would be reduced and sloughing would be inhibited. Additionally, it has been found that as the time for ignition is decreased, the amount of fuel required for ignition is also decreased.
Thus, decreasing ignition time can enhance the economics of the retorting operation.
However, in addition to providing a minimized ignition time, the ignition process must also be safe.
When oil shale is heated above the pyrolysis temperature of kerogen, the kerogen decomposes to give off combustible decomposition products. The pyrolysis temperature of kerogen is considered the ignition temperature of oil shale and has been found for certain oil shales to be from about 650.degree. F. to about 700.degree. F.
Oxygen provided in hot ignition gases can combine with such kerogen decomposition products, thereby burning the oil shale. Burning oil shale in situ adds energy to the process and, thus, the time it takes to ignite the retort can be decreased by supplying an ignition gas that contains oxygen.
Until the ignition process is completed and the combustion zone being formed extends across the entire horizontal section of the retort, portions of the surface of the fragmented mass of formation particles are below the ignition of pyrolysis temperature of kerogen in the oil shale. As a result, some of the hot ignition gas does not come into contact with oil shale heated above its ignition temperature. Oxygen in this portion of the ignition gas is not consumed and channels down the retort. Thus, during ignition, when the oxygen concentration of the ignition gas is increased, the concentration of oxygen in retort off-gas initially increases.
Since oxygen can combine in the retort with combustible gases produced by heating of oil shale, an increase in the oxygen concentration of the hot ignition gas can result in retort off-gas comprising sufficient oxygen that it is explosive. Having an explosive off-gas is unsafe.
A process is needed, therefore, that provides an optimum concentration of oxygen in hot ignition gas for minimizing retort ignition time, while at the same time providing that the concentration of oxygen in the retort off-gas remains below a value selected with regard to safety.